In cellular digital radiocommunications systems, each cell, which covers a given geographical zone, is provided with a base transceiver station (referred to merely as a "base station") serving as an interface between firstly a mobile station located substantially inside the geographical zone covered by the cell, and secondly either the management network for managing the radiocommunications system, or else another mobile station.
Each base station is associated with at least one transmission frequency in the down direction (base to mobile); a corresponding transmission frequency in the up direction corresponds to the down frequency. The expression "transmission frequency" is used below to designate a down transmission frequency. No further mention is made of up transmission frequencies, it being understood that there is one up transmission frequency per down transmission frequency.
In systems using the TDMA principle, each transmission frequency is subdivided into frames that are repeated following a periodic pattern. Each frame is itself subdivided into time slots, a periodically-repeated time slot of given rank constituting a transmission channel.
The transmission channels include at least one signalling channel or beacon path over which signalling messages are transmitted, and traffic channels for transmitting useful data (speech or data in other forms). In general, the signalling channel corresponds to the first time slot of each frame on a specific transmission frequency referred to as the "beacon frequency". The beacon frequency is characteristic of the cell. For example, the rank of the first time slot is identified by the number 0 in systems complying with the GSM standard, and by the number 1 in systems complying with the future TETRA standard.
The signalling channel is itself constituted by a plurality of sub-channels, each sub-channel also being repeated following a periodic pattern that is specific to it.
Therefore, both signalling messages and useful data can be transmitted on the beacon frequency. When there is a very large amount of call traffic in the cell, a plurality of other transmission frequencies may be used to satisfy needs, i.e. to transmit useful data.
In certain systems, e.g. systems complying with the future TETRA standard, an auxiliary beacon frequency is provided in addition to the main beacon frequency so that signalling messages can be transmitted when traffic is such that the main beacon frequency no longer suffices, and/or when, for security reasons, transmitting all the signalling messages on the same frequency is to be avoided.
The signalling messages or the useful data are transmitted over the appropriate channels in the form of packets or bursts.
When a mobile station, which is initially in standby mode, seeks to be connected to the management network for managing the radiocommunications system, it must determine the beacon frequency of the cell in which it is located. Since TDMA is being used, the mobile station must then become synchronized with the base station of the cell so that it uses the proper channel, i.e. the proper time slot.
In the same way, when a mobile station located in a given cell is inactive with respect both to transmission and to reception and optionally if the signal received from the base station of the cell in which the mobile station is located does not have sufficient power or a sufficient bit error rate (BER), the mobile station must "listen" to the beacon frequencies of the neighboring cells so as to measure the power (or the BER) of the signal received on those frequencies. The mobile station must also be synchronized with the neighboring cells so as to prepare for the possibility of handover being performed from the cell in which it is located to the cell having a beacon frequency that is received, for example, at a power level that is higher than the power level of the beacon frequency of the cell in which the mobile station is located.
For that purpose, it is essential for the base station to transmit continuously on the beacon frequency.
This poses no problems when the signalling channel and all of the traffic channels carried by the beacon frequency are used.
Unfortunately, e.g. when there is a small amount of call traffic, certain traffic channels might not be used, or even, at certain times, the signalling channel might not be used to 100% of its capacity, so that certain time slots on that channel are not used. In which case, "dummy" bursts are transmitted. The structure of such bursts is predetermined and is such that they cannot be confused with signalling bursts, so that when it hears them, the mobile station knows that they are not signalling bursts.
Such dummy bursts are transmitted by the base station under the control of the network, as soon as a channel or a time slot of a channel (sub-channel) is not used on the main beacon frequency, or, where applicable, on the auxiliary beacon frequency.
To become synchronized with the base station of a determined cell, once the beacon frequency of that cell has been detected, the mobile station must search for a "synchronization" burst.
The synchronization burst is transmitted following a periodic repetition pattern over a sub-channel of the beacon path. In a system complying with the GSM standard, the transmission period of the synchronization burst is about 0.04 seconds (s); in a system complying with the future TETRA standard, the transmission period of the synchronization burst is about 1 s.
Therefore, at certain times, as a function of the operating mode to which it has been programmed, the mobile station seeking to become synchronized "listens" to what is transmitted on the beacon frequency. During the entire "synchronization acquisition" period, the mobile station cannot transmit signalling data or useful data to the new cell, nor can it receive signalling data or useful data therefrom.
Therefore, during an access procedure, a considerable amount of time might be required for synchronization acquisition. During a call reestablishment procedure (e.g. a handover procedure in systems complying with the GSM standard), the call in progress might be interrupted so long as synchronization is not acquired, it being possible for the synchronization acquisition time to be longer than 20 s (in systems complying with the future TETRA standard).
Naturally, both of these situations are highly detrimental.